Vehicle transmissions are designed to transmit rotational force or torque from an engine or other prime mover to the drive wheels of the vehicle in order to propel the vehicle at a relatively wide range of output speeds. The engine includes a rotatable crank shaft or output shaft that can be selectively connected and disconnected from a transmission input shaft depending upon the desired transmission operating state. When the vehicle is configured with a manual transmission, a foot-operated clutch pedal positioned within the vehicle interior can be selectively actuated in order to allow the driver to shift gears and/or place the transmission in neutral. In an automatic transmission, this connection is provided automatically via a hydrodynamic torque converter assembly.
A hydrodynamic torque converter assembly, hereinafter referred to simply as a torque converter, typically includes an impeller or a pump, a turbine, and a stationary portion or a stator. The torque converter is filled with a viscous fluid or oil. The pump, which can be bolted to a rotating flywheel portion or other rotating portion of the engine in order to continuously rotate at engine speed, discharges a supply of fluid to the turbine. A stator is installed and shaped in such a way as to redirect the fluid discharged from the turbine back into the pump. The turbine in turn is connected to the transmission input shaft. The torque converter as a whole thus enables a variable fluid coupling effect to occur automatically between the engine and the transmission, allowing the vehicle to slow to a stop without stalling, while also allowing torque multiplication to occur at lower vehicle speeds.
In some torque converter designs a lock-up torque converter clutch or TCC is used to selectively join or lock the rotating pump to the rotating turbine above a calibrated threshold lockup speed. Below the threshold lockup speed, the torque converter is uniquely configured to allow an increasing amount or level of slip to occur across the torque converter as vehicle speed decreases, ultimately reaching a maximum slip level when vehicle speed reaches zero. Regardless of whether a TCC is used, this variable slip capability allows the engine to continue to rotate when the vehicle is idling in certain transmission settings or states, e.g., in park (P), neutral (N), or when in a drive (D) state while the vehicle is at a standstill, a condition or state collectively referred to hereinafter as “neutral idle (NI)”. However, although such variable slip capability is invaluable to the effective operation of a conventional automatic transmission, slip inherently results in some portion of total available power to be lost between the engine and the transmission due to viscous friction of the transmission and other vehicle components.